The invention relates to a capillary valve that can be pulsed and to the use thereof.
Generally so far, a gas to be analyzed is introduced into the ion source of a mass spectrometer in a continuous or pulsed fashion. In such an arrangement, a supply line (for example, the end of a gas chromatographic capillary) extends into the ion source which, may be of closed design (for example, many CI- or EI ion sources for quadrupole or sector field mass spectrometers) or of open design (for example, many ion sources for flight time mass spectrometers). In ion sources of closed design, an area of the ion source is “flooded” by the gas supplied, that is, the atoms or molecules entered partially bounce onto the ion source walls before they are ionized and detected in the mass spectrometer. The open design of many ion sources for the TOF mass spectrometer favors the use of atom-or molecular beam techniques. In that case, a relatively directed gas jet is conducted through the ion source, such that, ideally, it interacts only very little with the components of the ion source.
For the flight time mass spectometery, effusive molecular beams [R. Zimmerman, H. J. Heger, A. Kettrup, U. Boesl, Rapid Communic. Mass Spectrom. 11 (1997) 1095] as well as skimmed [R. Tembreull, C. H. Sin, P. Li, H. M. Pang, D. M. Lubman; Anal. Chem. 57 (1985) 1186] and unskimmed [R. Zimmermann, H. J. Heger, E. R. Rohwer, E. W. Schlag, A. Kettrup, U. Boesl, Proceedings of the 8th Resonance Ionization Spectroscopy Symposium (R1S-96), Penn State College 1996, AIP Conference Proceedings, 388, AIP-Press, Woodbury, N.Y. (1997) 1119] supersonic molecular beams are used (in each case either pulsed or continuous (cw)). Supersonic molecular beam inlet systems permit the cooling of the analysis gas in the vacuum by adiabatic expansion. It is however a disadvantage that, with conventional systems, the expansion must take place relatively far away from the ionization location. Since the density of the gas expansion beam (and consequently, the ion yield for a certain ionization volume) decreases with the distance from the expansion nozzle in square, the achievable sensitivity is limited.
Effusive molecular beam inlet systems do not permit cooling of the sample. However, gas inlet systems for effusive molecular beams can be so constructed that, by way of a metallic needle, which extends to the center of the ion source, the discharge gas is guided directly to the ionization location. A certain electric potential is applied to that needle so as to avoid disturbance of the withdrawal fields in the ion source. The needle must be heated to relatively high temperatures in order to prevent condensation of non-volatile analyte molecules in the needle. Care must be taken that the coldest point is not at the needle tip. The necessary heating of the needle is problematic since the needle must be insulated with respect to the other components of the structure (for example, by a ceramic transition member). Electric insulators are generally also thermal insulators and therefore provide for only a very low heat flow for example from the electrically heated supply duct to the needle. Heating by electric heating elements or an IR radiator is also difficult since the needle extends between the withdrawal plates of the ion source.
The selectivity of the resonance ionization with lasers (REMPI) depends on the inlet system used because of the different cooling properties. Besides the effusive molecular beam-inlet system (EMB), which may be used among others for the detection of complete classes of substances, it is possible, with the use of a supersonic molecular beam inlet system (jet), to ionize in a highly selective and partially even isomer-selective manner. With the common supersonic gas nozzles, which have been developed for spectroscopic experiments, the utilization of the sample amount (that is, the achieveable measuring sensitivity) is not a limiting factor. Furthermore, the existing systems are not designed to avoid memory effects. For the use of REMPI-TOFMS-spectrometers for analytical applications, the development of an improved jet inlet technique would be advantageous. Care is to be taken that the valves are made of an inert material in order to avoid memory effects or chemical decomposition (catalysis) of the sample molecules. Furthermore, for analytical applications, the valves should have no dead volumes. It is also necessary that the valves can be heated to temperatures above 200° so that also relatively involatile compounds of the mass-area >256 amu are accessible. Furthermore, the sensitivity with respect to the effusive inlet technique should essentially not be detrimentally affected by the jet arrangement. This can be achieved mainly by a more effective utilization of the introduced samples in comparison with conventional jet arrangements.
This increase is achieved in that each laser pulse reaches a relatively large part of the sample. Since the excitation volume is predetermined by the dimensions of the laser beam (a widening of the laser beam would reduce the REMPI effective cross-section, which for example with a two-photon ionization corresponds to the square of the laser intensity) it must be tried to optimize the spatial overlap of molecular beam and laser beam. This can be realized for example by a pulsed inlet. Boesl and Zimmermann et al. have presented for example a heatable pulsed jet valve for analytical applications for example for a gas chromatography-jet-REMPI-coupling with minimized dead volume [DE 195 39 589.1].
Pepich et al. presented a GC supersonic molecular beam-coupling for the laser-induced fluorescence spectroscopy (LIF), wherein, with the pulsed inlet and a sample compression, the duty cycle can be increased in comparison with the effusive inlet [B. V. Pepich, J. B. Callis, D. H. Burns, M. Grouterman, D. A. Kalman, Anal. Chem. 58(1986) 2825].
All pulsable inlet systems described so far have the following disadvantages. Because of their geometric dimensions, they need large samples and impulse gas volumes in order to facilitate an adiabatic cooling. Their geometry does not permit the valve outlet to be placed near the ionization location. Because they are mechanical devices, they have long opening times and therefore generate a relatively large gas pulse, which results in a heavy load on the evacuation system.
It is the object of the invention to construct a pulsable capillary valve in such a way that it is suitable for small sample amounts and to indicate a use for such a capillary valve.